In an oil well environment, downhole fluid flow through the well bore is developed by one or more production zones at levels where fluids in the earth formation enter the well bore under pressure and can include hydrocarbons, water, gas and dissolved gas or a combination of those fluids. The fluid entry to the well bore is through perforations along the length of the casing or liner and, in some wells, the fluid flow can be from one or more production zones at different depths in the well bore.
One of the matters of interest to the operator is the fluid velocity of the fluid flow and the type of fluid, i.e. hydrocarbon, gas, water, or mixture of fluids.
At present, there are a variety of oil field flowmeters in use which utilize spinning impellers where the rotational speed of the impeller is proportional to the velocity of fluid flow. In use, such flowmeters can be utilized to obtain data for production profiles, i.e. which zones produce fluid as well data reflecting a change in a production velocity profile over period of time. Flowmeters are also used for monitoring fluid injection in secondary recovery operations and for determining thief zones, as well as other specialized operations.
A continuous flowmeter is generally utilized for high volume flow and its response is principally affected by viscosity changes of fluids. A packer flowmeter is used in low volume flow applications. Both types of devices require a minimum flow rate or velocity of fluid for operation. The fluid must overcome the friction effects in the flowmeter impeller system.
A packer tool or basket flowmeter permits measurement of total flow but requires a packer device which can obtain a good pack off and principally works better with liquids. The tool must be set and reset for different levels of production.
A continuous flowmeter can make measurements while the tool is moving thereby providing a continuous recording of flow rates. Since only a percentage of the fluid passes through the tool, centering of the tool in the well bore is important to obtain good measurements.
With all rotating impeller flowmeters in addition to minimum velocity to maintain motion of the impeller, the impeller is a mechanically functioning device which is subject to friction effects and is subject to malfunctions. Additionally, impellers because of their high dependence on velocity do not distinguish between the type of fluids i.e. water, gas, hydrocarbon.
For a more detailed discussion of the prior art reference can be made to the article entitled "State of the art simultaneous downhole flow rate and pressure measurement equipment" by G. W. Haws, et.al. in the SPE Production Engineering, November, 1991, pages 427-433.
In an effort to avoid rotating impellers, acoustic flowmeters have been proposed for gas wells. An acoustic flowmeter measures the travel time of acoustic energy between transducers. Reference can be made to the article "Acoustic Flowmeter Field Test Results" by Richard A. McBane in SPE Production Engineering, February, 1991, pages 49-56 for a more detailed description.
Also, other tools can be used in combination with a flowmeter, such as a casing collar locator, a thermometer, a caliper, a manometer or a gradiomometer.